1. Field of the Invention
The present invention relates to a method of solder pre-coating in forming a solder pre-coat layer on an electrode on a circuit board and to a solder pre-coated circuit board having an electrode pre-coated with solder.
2. Description of the Prior Art
As a method of previously coating solder on an electrode on a circuit board on which electronic components are to be mounted, one using a tackiness adding compound is well known (ref. Japanese Patent No. 2,592,757). This is a method in which a solder pre-coat layer is formed on an electrode by first adding tackiness on the surface of the electrode by making a tackiness adding compound react with the electrode consisting mainly of copper , followed by attaching solder powder on the tackiness added electrode, and melting the attached solder. This method permit one, and to form a solder pre-coat with a uniform thickness by supplying solder only on the electrode portion of the surface of a circuit board.
On the other hand, there is a case in which a solder pre-coat is partially formed (so-called partial solder pre-coat) only on a specific area of an electrode or on a specific electrode among many electrodes formed on a circuit board, for instance, a specific electrode to which metal bumps of electronic components are to be joined. In such a case, too, it is preferable to apply the above-described method of treatment with tackiness adding.
However, in order to form a solder pre-coat on a predetermined portion or an electrode only out of a plurality of electrodes by a solder pre-coating method using the prior art tackiness adding treatment, it was necessary to mask electrode portions other than the predetermined ones with a protective film and the like for each individual circuit board. Therefore, the prior art method caused an increase in the cost due to complication of the solder pre-coating process and the possibility of damaging reliability of the circuit board due to otherwise unnecessary protective film remaining in the product.
Furthermore, a new fact has been found through investigation and experiments by the present, inventor on the above-mentioned tackiness adding compound that sufficient tackiness could not be obtained by making the above-mentioned tackiness adding compound react with a gold film which has a high degree of surface cleanliness.
Now, therefore, it is an object of the present invention to provide a simple method of forming a solder pre-coat layer with which a high quality partial solder pre-coat layer can be formed.
The solder pre-coating method of the present invention includes:
dry etching a surface of a circuit board on which a plurality of electrodes having a gold film on its surface by covering the surface of the circuit board with a template having an opening, thereby cleaning the surface of the gold film other than predetermined portion of the electrodes;
adding tackiness on the surface of a predetermined portion of the electrode by making a tackiness, adding compound react with the surface of the electrode after cleaning;
attaching solder powder to the tackiness added electrode; and
forming a solder pre-coat layer only on a predetermined portion of the electrode by melting the solder powder by heating.
Other solder pre-coating method of the present invention includes:
forming a metal film containing at least one of copper or nickel on a surface of a predetermined portion of an electrode by covering with a template having an opening a circuit board on which a plurality of electrodes having a gold film on their surfaces;
adding tackiness on the surface of the metal film by making a tackiness adding compound react with the electrode surface;
attaching solder powder to the tackiness added electrode; and
forming a solder pre-coat only on the predetermined portion of the electrode by melting the solder powder by heating.
According to the present invention, among the plurality of electrodes covered with a gold film having a surface of relatively low degree of cleanliness, those electrode portions which should not be pre-coated with solder can be cleaned by dry etching and the impurities on the surface are removed. Consequently, the, tackiness adding compound reacts only with the electrode portions having much impurities and selectively adds tackiness only to those electrode portions. Partial solder pre-coating can be performed by subsequently attaching solder powder on the tackiness added electrode portions and melting the solder powder by heating.